Genetics

Vocabulary Trait Hybrid Gene Specific characteristic Offspring of parents with different traits Gene Factors that determine traits

Allele The different forms of a gene Segregation Separation of alleles

Probability Punnett square Homozygous The likelihood an event will happen Punnett square A diagram to show the results of a genetic cross Homozygous Have 2 identical alleles

P = Parental generation F1 = First offspring produced F2 = Second offspring produced

Heterozygous Phenotype Genotype 2 different alleles Physical characteristics Genotype Genetic makeup

Independent assortment Carrier Carries a trait but doesn’t exhibit it Independent assortment Genes do not influence each other’s inheritance Incomplete dominance When one allele is not completely dominate over another

Codominance Multiple alleles Polygenic traits Both alleles contribute to the phenotype Multiple alleles When genes have more than 2 alleles Polygenic traits Traits controlled by two or more genes

Gregor Mendel Known as the “Father of Genetics” Genetics = the study of heredity Carried out his work with ordinary garden peas.

Mendel summarized his findings into 2 Laws: 1) Law of segregation 2) Law of independent assortment

Law of Segregation States that every individual possesses a pair of alleles (assuming diploidy) for any trait Each parent passes a randomly selected copy (allele) of only one of these to its offspring The offspring then receives its own pair of alleles for that trait. Whichever of the two alleles in the offspring is dominant determines how the offspring expresses that trait (e.g. the color of a plant, the color of an animal's fur, the color of a person's eyes).

Law of Segregation Proven by meiosis The paternal and maternal chromosomes get separated and the alleles with the traits of a character are segregated into two different gametes.

Law of Independent Assortment States that separate genes for separate traits are passed independently of one another from parents to offspring Occurs during Metaphase I of Meiosis The “double-file” line + crossing over

Punnett Squares A capital letter represents the dominant allele A lowercase letter represents the recessive allele Use the first letter of the dominant trait Homozygous Dominant = BB Homozygous Recessive = bb Heterozygous = Bb

Probability & Segregation One fourth (1/4) of the F2 plants have two alleles for tallness (TT). 2/4 or 1/2 have one allele for tall (T), and one for short (t). One fourth (1/4) of the F2 have two alleles for short (tt)

Using Punnett Square to Prove Independent Assortment Mendel performed a two-factor cross Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY) with true-breeding plants that produced wrinkled green peas (genotype rryy). All of the F1 offspring produced round yellow peas (RrYy) The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y).

The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation

Incomplete Dominance

Codominance

Human Heredity Karyotype A picture of the arrangement of chromosomes

Sex Chromosomes Autosomes X & Y Males = XY Females = XX Males determine the sex of the offspring Autosomes The other 22 pair of chromosomes

A pedigree chart shows the relationships within a family.

Human Traits A square represents A circle represents a male. a female. A vertical line and a bracket connect the parents to their children. A horizontal line connecting a male and a female represents a marriage. A shaded circle or square indicates that a person expresses the trait. A circle or square that is not shaded indicates that a person does not express the trait. This drawing shows what the symbols in a pedigree represent. Copyright Pearson Prentice Hall

Recessive Alleles The presence of a normal, functioning gene is revealed only when an abnormal or nonfunctioning allele affects the phenotype. Many disorders are caused by autosomal recessive alleles

Dominant Alleles The effects of a dominant allele are expressed even when the recessive allele is present. Two examples of genetic disorders caused by autosomal dominant alleles are achondroplasia and Huntington disease.

Codominant Alleles Sickle cell disease is a serious disorder caused by a codominant allele. Sickle cell is found in about 1 out of 500 African Americans.

People who are heterozygous for the sickle cell allele are generally healthy and they are resistant to malaria. An individual with both normal and sickle cell alleles has a different phenotype—resistance to malaria—from someone with only normal alleles

If two copies of an autosomal chromosome fail to separate during meiosis, an individual may be born with three copies of a chromosome. Down syndrome involves three copies of chromosome 21.

Sex-Linked Genes The X chromosome and the Y chromosomes determine sex. Genes located on these chromosomes are called sex-linked genes. More than 100 sex-linked genetic disorders have now been mapped to the X chromosome

For a recessive allele to be expressed in females, there must be two copies of the allele, one on each of the two X chromosomes. Males have just one X chromosome. Thus, all X-linked alleles are expressed in males, even if they are recessive.